Environmental Engineering Reference
In-Depth Information
To calculate the hydraulic horsepower (WHP) using flow in gpm and
head in feet, use the following formula for centrifugal pumps:
Flow (gpm)Head (ft)SpecificGravity
39
×
×
60
WHP
=
(3.15)
Note:
3960 is derived by dividing 33,000 ft-lb by 8.34 lb/gal = 3960.
3.2.8.2 Brake Horsepower
Key Points:
(1)
Water horsepower
(WHP) is
the power necessary to lift the water to the
required height;
brake horsepower
(BHP)
is the horsepower applied to the pump;
(3)
motor horsepower
is the horsepower
applied to the motor; and (4)
efficiency
is
the power produced by the unit divided by
the power used in operating the unit.
A water pump does not operate alone. It is
driven by the motor, and electrical energy
drives the motor. Brake horsepower (BHP) is
the horsepower applied to the pump. The BHP
of a pump equals its hydraulic horsepower
divided by the efficiency of the pump. Note
that neither the pump nor its prime mover
(motor) is 100% efficient. Both of these units
experience friction losses; more horsepower will have to be applied to
the pump to achieve the required amount of horsepower to move the
water, and even more horsepower must be applied to the motor to get the
job done (Hauser, 1993). The formula for BHP is:
Flow (gpm)Head (ft)
×
×
SpecificGravity
BrakeHorsepower (BHP)
=
(3.16)
3960
×
Efficiency
3.2.9 specific speed
The capacity of flow rate of a centrifugal pump is governed by the
impeller thickness (Lindeburg, 1986). For a given impeller diameter, the
deeper the vanes, the greater the capacity of the pump. Each desired flow
rate or desired discharge head will have one optimum impeller design.
The impeller that is best for developing a high discharge pressure will
have different proportions from an impeller designed to produce a high
flow rate. The quantitative index of this optimization is called the
spe-
ciic speed
(
n
s
)—the higher the specific speed of a pump, the higher its
efficiency. The specific speed of an impeller is its speed when pumping 1
gpm of water at a differential head of 1 ft. The following formula is used
to determine specific speed (where
h
is at the best efficiency point):
rpm
×
q
0.5
n
s
=
(3.17)
075
.
h
where:
rpm = revolutions per minute.
q
= flow (in gpm).
h
= head (in ft).
